An example of known gas turbines is shown, for example, in FIG. 1. FIG. 1 shows a sectional view of a gas turbine at the outer gas path. The gas turbine has a casing 1 having a bore 2 and a bleed duct 3. This substantially horizontally extending casing 1 separates the lower region (primary gas path 10) from the upper region (secondary gas path 12). In this view, the working fluid (here gas) flows from left to right. A variable guide vane 4 has a trunnion 5, a rotary plate 6, and a guide vane airfoil 7. Trunnion 5 is disposed in bore 2 and retained by a guide sleeve 8 disposed in bore 2. A sealing ring 9 is disposed between bore 2 and rotary plate 6. Guide vane airfoil 7 has a leading edge 14 and a trailing edge 16 in accordance with the prior art. Leading edge 14 mostly does not extend beyond rotary plate 6. In contrast, trailing edge 16 extends beyond rotary plate 6, so that a short flag 18 is created. The point at which flag 18 and trailing edge 16 meet is referred to as flag corner 20. An inlet opening 22 of bleed duct 3 is disposed downstream of this known trailing edge of guide vane airfoil 7. This means that in the prior art, the beginning 24 of inlet opening 22 is located aft of flag corner 20. In this connection, it is irrelevant whether the overhang beyond rotary plate 6 is small, or whether the beginning 24 of inlet opening 22 is located further downstream.
A first outer surface 30 of the annular space can be seen upstream of bore 2. First outer surface 30 is located at a first radius r1 from main shaft 60 (see FIG. 3) of the gas turbine. A second outer surface 32 can be seen between bore 2 and inlet opening 22 of bleed duct 3. A third outer surface 34 of the annular space can be seen downstream of inlet opening 22, the third outer surface mostly being disposed in alignment with second outer surface 32 (as indicated by connecting line V). In the prior art, there is a continuous transition between these three outer surfaces; i.e., the annular space is mostly cylindrical or conical in shape at these locations.
Such bleed ducts 3 serve to direct fluid from primary gas path 10 to secondary gas path 12. It must be ensured that sufficient fluid is directed through bleed duct 3 at sufficient pressure.